1. Field of Invention
The present invention relates in general to delta-sigma modulators and, in particular, to noise shaping circuits and methods with feedback steering overload compensation and systems using the same.
2. Background of Invention
Delta-sigma modulators are particularly useful in digital to analog converters (DACs) and analog to digital converters (ADCs). Using oversampling, a delta-sigma modulator spreads the quantization noise power across the oversampling frequency band, which is typically much greater than the input signal bandwidth. Additionally, the delta-sigma modulator performs noise shaping by acting as a highpass filter to the quantization noise; most of the quantization noise power is thereby shifted out of the signal band.
The typical delta-sigma modulator in an ADC includes an input summer which sums the analog input signal with negative feedback, an analog linear (loop) filter, a quantizer, and a feedback loop with a digital to analog converter (feedback DAC) coupling the quantizer output and the inverting input of the input summer. A delta-sigma DAC is similar, with a digital input summer, a digital linear filter, a digital feedback loop, a quantizer, and an output DAC at the modulator output: In a first order modulator, the linear filter comprises a single integrator stage; the filter in higher order modulators normally includes a cascade of a corresponding number of integrator stages. Higher-order modulators have improved quantization noise transfer characteristics over modulators of lower order, but stability becomes a more critical design factor as the order increases. For a given topology, the quantizer is either a one-bit or a multiple-bit quantizer.
One cause of instability in digital delta-sigma modulators is input overload. For example, input overload occurs when the gain of the input data is greater than one, when a digitized squarewave with significant Gibbs overshoot is received at the modulator input, or when a bad stream of data is fed from a preceding interpolator. Single-bit delta-sigma modulators are notoriously susceptible to input overload. Multiple-bit delta-sigma modulators are less susceptible to input overload, although overload will still often occur when the input stream approaches its maximum positive and negative levels.
Current techniques for handling overload in delta-sigma modulators are relatively complex and require detection of overload conditions and subsequent resetting or limiting of the modulator circuitry to avoid saturation and instability. However, modulator overload remains an important problem that must be addressed, especially in higher order modulators that provide higher quality noise shaping. Modulator overload is particularly troublesome in audio applications, in which an unstable modulator causes extremes in the output signal that damage the following processing stages and/or result in an unpleasant audible output to the listener.
According to the inventive concepts, methods and circuits are disclosed which provide noise shaper immunity to input overload. One representative embodiment of these concepts is a noise shaper including a first filter for noise shaping an input signal under normal operating conditions and a second filter that is stable under overload conditions. A quantizer responds to the sum of the outputs of the first and second filters. Signal steering circuitry steers feedback from the output of the quantizer to inputs of the first and second filters to maintain stability of the first filter under the overload conditions.
Circuits and methods embodying the inventive concepts directly address the problem of noise shaper input overload. When an overload condition occurs, the overload loop receives and bears the increased energy load while the energy being passed by the primary (high quality) noise shaping loop is sustained at a level to maintain primary loop stability. When the overload condition ceases, the primary loop resumes passing the majority of the energy and continues to provide the high quality noise shaping operation . The present invention does not require additional circuitry to either detect overload conditions or reset the noise shaper circuitry to avoid saturating the noise shaper output. Additionally, brief deviations of the input stream outside of the normal maximum limits of the noise shaper input do not substantially disrupt noise shaper operation. These circuits and methods are particularly useful in audio applications in which noise shaper overload causes damage in the following processing stages, such as the audio amplifiers and speakers, and even produce an audible output injurious to the hearing of the listener.